In order to perform a high frequency (HF) ultrasound scan of the cornea, it is necessary to provide a conductive medium between the moving transducer which generates the ultrasound waves and the surface of the cornea under examination. The preferred medium is a balanced salt solution which has the same osmolarity as the tears in the eye in order to prevent corneal edema.
At present, there are two principle types of HF ultrasound scanning apparatus. In an apparatus that is designed to obtain highly accurate biometric data for the purposes of producing computer-generated images of the structure and topography of the cornea and adjacent surfaces, the subject is required to lie supine on an examination table so that the apparatus can be positioned over the patient's upwardly facing eyes. In order to establish a liquid bath above the cornea into which the tip of the transducer can be immersed, a water-tight dam or drape must be established around the eye. This is typically done by securing the interior surface of an annular sheet of flexible polymeric material to the skin around the subject's eye using a physiologically compatible adhesive. The exterior periphery of the flexible polymeric dam must then be retained in an upstanding and outwardly extending position, as by securing it in a stationary double-ring clamp. The subject's eye is treated with an anesthetic and the eyelids are maintained in an open position by means of a lid speculum. Thereafter, the dam around the eye is filled with a normal saline solution to a depth of approximately 2 cm above the cornea so that the movable transducer tip can be immersed for scanning movement beneath the surface of the saline bath. The procedure is described in more detail in U.S. Pat. Nos. 5,293,811 and 5,369,454.
The length of time required to complete the scanning depends upon the nature and extent of the data required to prepare the images. In some case, only a few minutes is required to complete the scan, while the time required to prepare the subject for the procedure is much more extensive. Moreover, once the procedure has been completed, the dam must be pulled away from the subject's skin and the adhesive material removed. In most cases, the same procedure is repeated on the other eye, and the level of anxiety and discomfiture of the subject may be increased. The use of a flexible dam that has to be secured to the epidermal area around the eye necessarily requires a substantial amount of balanced saline solution and requires more time for clean-up and leads to inevitable spills of liquid on the subject, the technician and/or examining room equipment.
In a second type of apparatus that is in common use, the subject is seated upright in a chair with his/her head tilted back against a headrest; the subject's eye is anesthetized and the eyelids retained in an open position by a lid speculum; and an eye cup is placed in direct contact with the surface of the eyeball. The eye cup is filled with a balanced saline solution and a transducer attached to a flexible cable is manually inserted into the solution and moved to obtain the data required for the scan. It will be understood that this apparatus and method is far less capable of generating data from the whole cornea for producing biometric images. It is also somewhat more stressful for the subject, since despite the anesthetic, the presence of the eye cup in the eye induces a higher level of anxiety.
It is therefore an object of this invention to provide a method and apparatus for the high frequency ultrasound scanning where the human cornea is immersed in a liquid transmission medium that does not require adhesives or other means for semi-permanently attaching retaining means to contain the liquid medium.
It is another object of this invention to provide a method and apparatus for accomplishing ultrasound corneal scans that is more comfortable and less stressful for the subject than the procedures that are currently available.
It is yet another object of this invention to provide an apparatus in which the subject can sit comfortably and properly position his/her own eye in an eye cup containing the liquid medium through which the ultrasound waves are transmitted to accomplish the corneal scan.
A further object of the invention is to provide an apparatus in which the position of the subject's cornea with respect to the eye cup can be monitored by a video image employing a camera having a lens proximate the bottom of the eye cup.
Another object of this invention is to provide for multi-planar, spiral or meridional data collection for use in three-dimensional reconstruction and for volume measurement.
It is also an object of the invention to provide an improved method and apparatus for more efficiently accomplishing ultrasonic corneal scans that requires minimal preparation of the subject and less time to complete the preparation and the scan.
Further objects of the invention are to provide an improved apparatus and method employing a relatively inexpensive disposable and sterilizable eye cup that is discarded after use by each subject and that is replaced with a fresh sterile eye cup.